The present invention relates to ferromagnetic thin-film structures exhibiting relatively large response magnetoresistive characteristics and, more particularly, to such structures used for the storage and retrieval of digital data.
Many kinds of electronic systems make use of magnetic devices including both digital systems, such as memories, and analog systems such as magnetic field sensors. Digital data memories are used extensively in digital systems of many kinds including computers and computer systems components, and digital signal processing systems. Such memories can be advantageously based on the storage of digital symbols as alternative states of magnetization in magnetic materials provided in each memory storage cell, the result being memories which use less electrical power and do not lose information upon removals of such electrical power.
Such memory cells, and magnetic field sensors also, can often be advantageously fabricated using ferromagnetic thin-film materials, and are often based on magnetoresistive sensing of magnetic states, or magnetic conditions, therein. Such devices may be provided on a surface of a monolithic integrated circuit to provide convenient electrical interconnections between the device and the operating circuitry therefor.
Ferromagnetic thin-film memory cells, for instance, can be made very small and packed very closely together to achieve a significant density of information storage, particularly when so provided on the surface of a monolithic integrated circuit. In this situation, the magnetic environment can become quite complex with fields in any one memory cell affecting the film portions in neighboring memory cells. Also, small ferromagnetic film portions in a memory cell can lead to substantial demagnetization fields which can cause instabilities in the magnetization state desired in such a cell.
These magnetic effects between neighbors in an array of closely packed ferromagnetic thin-film memory cells can be ameliorated to a considerable extent by providing a memory cell based on an intermediate separating material having two major surfaces on each of which an anisotropic ferromagnetic memory thin-film is provided. Such an arrangement provides significant “flux closure,” i.e. a more closely confined magnetic flux path, to thereby confine the magnetic field arising in the cell to affecting primarily just that cell. This result is considerably enhanced by choosing the separating material in the ferromagnetic thin-film memory cells to each be sufficiently thin. Similar “sandwich” structures are also used in magnetic sensors.
In the recent past, reducing the thicknesses of the ferromagnetic thin-films and the intermediate layers in extended “sandwich” structures, and adding possibly alternating ones of such films and layers, i.e. superlattices, have been shown to lead to a “giant magnetoresistive effect” being present in some circumstances. This effect yields a magnetoresistive response which can be in the range of up to an order of magnitude or more greater than that due to the well known anisotropic magnetoresistive response.
A memory cell based on the “giant magnetoresistive effect” can be provided by having one of the ferromagnetic layers in the “sandwich” construction being prevented from switching the magnetization direction therein from pointing along the easy axis therein in one to the opposite direction in the presence of suitable externally applied magnetic fields while permitting the remaining ferromagnetic layer to be free to do so in the same externally applied fields. In one such arrangement, a “spin-valve” structure is formed by providing an antiferromagnetic layer on the ferromagnetic layer that is to be prevented from switching in the externally applied fields to “pin” its magnetization direction in a selected direction. In an alternative arrangement often termed a “pseudo-spin valve” structure, the ferromagnetic layer that is to be prevented from switching in the externally applied fields is made sufficiently thicker than the free ferromagnetic layer so that it does not switch in those external fields provided to switch the free layer.
An alternative digital data bit storage and retrieval memory cell suited for fabrication with submicron dimensions can be fabricated that provides rapid retrievals of bit data stored therein and low power dissipation by substituting an electrical insulator for a conductor in the nonmagnetic layer. This memory cell can be fabricated using ferromagnetic thin-film materials of similar or different kinds in each of the magnetic memory films used in a “sandwich” structure on either side of an intermediate nonmagnetic layer which ferromagnetic films maybe composite films, but this intermediate nonmagnetic layer conducts electrical current therethrough based primarily on a quantum electrodynamic effect “tunneling” current, or spin dependent tunneling.
Operating magnetic fields imposed externally can be used to vary the angle of the magnetization vector with respect to the easy axis in the ferromagnetic films of these various kinds of memory cell devices, particularly the free layers. Such operational magnetic fields imposed externally can be used to vary the angle to such an extent as to cause switching of the layer magnetization vector between two stable states which occur for the magnetization being oriented in opposite directions along the easy axis of the layer, the state of the cell determining the value of the binary bit being stored therein. One of the difficulties in such memories is the need to provide memory cells therein that have extremely uniform switching thresholds and adequate resistance to unavoidable interjected magnetic field disturbances in the typical memory cell state selection scheme used. This externally applied operating fields scheme is based on selective externally imposed magnetic fields provided by selectively directing electrical currents over or through sequences of such cells so that selection of a cell occurs through coincident presences of such fields at that cell. Such a coincident interjected magnetic fields memory cell state selection scheme is very desirable in that an individual switch, such as that provided by a transistor, is not needed for every memory cell, but the limitations this selection mode imposes on the uniformity of switching thresholds for each memory cell in a memory make the production of high yields difficult.
Such memory cells, whether based on being operated by coincidently provided magnetic fields only or operated with supplemental switching transistors, can be modified to use Curie point or Néel point data storage, or writing, techniques based on the thermal pulse accompanying a current pulse provided in or near the cell. If such storing currents are established that are sufficient to heat these storage layers to the Curie temperature thereof, then much less magnetic field strength would be needed to change the magnetic states of the storage layers and the values of the storing currents could be much reduced to effectively avoid significant magnetization rotation thresholds in the device magnetic material layers.
Another alternative for “sandwich” structure magnetoresistive memory cells is to switch between the magnetic states of such memory cells operated by switching transistors to control the supplying of spin polarized electrical currents thereto to reverse the magnetization direction of a soft magnetic material layer therein in the absence of any externally applied magnetic field coincident therewith. A spin polarized electrical current has therein electrons flowing with their spins aligned in one direction predominating the number of electrons therein with spins in the opposite direction. Such spin injection currents with the spins of electrons therein predominating in one direction or the other lead to a corresponding spin injection torque on the device magnetic material layer magnetization sufficient to reverse the direction thereof as a device magnetic state change.
With or without the use of such thermal pulse techniques or the use of spin polarized electrical currents, however, as the dimensions of magnetoresistive elements in memory cells shrink in size to be on the order of a few nanometers, the problems that must be overcome become more difficult in order to make successful operating magnetoresistive memories having long data retention times and low error rates. These problems include (a) the increasing thermal instability of those cells therein intended not to have the magnetic states thereof switched in current operations because less thermal energy is needed to upset the magnetic states of smaller memory cells, (b) the needing of larger electrical current densities to overcome the greater demagnetization fields of smaller cells in operations to store information therein through switching between the magnetic states thereof which risk damage to the device or become impractical to generate in the device, and (c) the greater stray field interactions between adjacent memory cells on a common substrate.
A small magnetic material memory element with uniaxial anisotropy generally tends to preferentially align its magnetization with the uniaxial anisotropy axis in the absence of an externally applied magnetic field. As a function of the angle between the direction of the magnetization, and the direction of the easy-axis, the energy of the small element may be described as E=Keff sin2 where Keff is a constant related to the properties of the element ferromagnetic material and to the shape of the element, and t is the angle between the easy-axis and the direction of the magnetization in the element as shown for an elliptically shaped element in FIG. 1. The magnetization direction of the element can be reversed by applying an external magnetic field Ha=2Keff/Ms. The energy barrier needed to be overcome to reverse the magnetization of the element is related to the volume of the element by ΔE=Keff V. Note that application of a magnetic field less than Hc lowers the energy barrier by an amount 0.5* (2Keff/Ms−Ha)V if switching into the direction of the applied field. This reduction of the energy barrier in the direction of the applied field is useful for reducing the current required to reverse the free layer of a memory device using the spin-momentum transfer effect. At nonzero temperature, there is a finite probability that the magnetization of the element will spontaneously reverse. In a finite time interval Δt, the probability an element will reverse due to thermal activation is P(Δt, T, ΔE)=P0e−Δt/τ where an Arrhenius law relates the relaxation time constant to the energy barrier and temperature,
  τ  =            1              f        0              ⁢                  ⅇ                  Δ          ⁢                                          ⁢                      E            /                          k              B                                ⁢          T                    .      Here f0 is an attempt frequency of ˜109 Hz that is often considered a constant, and τ is a relaxation time. The energy barrier defining the thermal stability or data retention time of the element is labeled ΔE in FIG. 1, and it represents the amount of energy needed to be applied in the absence of thermal fluctuations (at absolute zero) to the ferromagnetic material element to reorient the magnetization from 0° to 180° or from 180° to 0°.
The expected probability to switch in a finite time period is illustrated in FIG. 2. The ratio ΔE/kBT=S is often called the stability factor, and it relates the zero temperature energy barrier to the thermal energy of the system. It is clear that S≧ln(f0τ) in order for the memory element to retain data for a time period of τ. For τ=10 years, this value is S=40 as shown in FIG. 2. Because the energy barrier is proportional to the volume of the element, maintaining a large stability factor with a small fixed volume requires an increase in Keff.
However, increasing Keff also increases the externally applied magnetic fields needed to be used for selecting the desired magnetic state in the memory element to represent current data therein, and so also the associated electrical currents by which the memory element is operated through generating such external fields, to impractical levels as indicated in FIG. 3. This figure shows, for an elliptically shaped, ferromagnetic material memory element with an arbitrarily selected size of 75 nm by 50 nm to thereby be a single domain element, the switching energy or stability factor versus the element layer thickness (and so effectively versus volume) and the resulting magnetic coercivity of that layer versus that thickness. A horizontal line in the figure at a switching energy value of 40 intersects the plot of the Keff V stability curve at a memory element layer thickness of about 2.5 nm which is shown by a vertical arrow to the plot of the coercivity curve and a subsequent horizontal arrow to result in a coercivity of about 200 Oe. Thus, an externally applied magnetic field on the order of this value is required to accomplish reversing the magnetization direction in the element. This shows the difficulty in using only an externally applied magnetic field to provide such switching as generating a field in excess of 100 Oe is difficult in monolithic integrated circuit based magnetoresistive memory devices. Thus, there is a desire for alternative, or supplemental methods, for storing information in magnetoresistance based memory cells.